全文获取类型
收费全文 | 1320篇 |
免费 | 156篇 |
国内免费 | 108篇 |
专业分类
化学 | 314篇 |
晶体学 | 4篇 |
力学 | 144篇 |
综合类 | 4篇 |
数学 | 111篇 |
物理学 | 1007篇 |
出版年
2024年 | 1篇 |
2023年 | 14篇 |
2022年 | 23篇 |
2021年 | 17篇 |
2020年 | 25篇 |
2019年 | 31篇 |
2018年 | 34篇 |
2017年 | 48篇 |
2016年 | 59篇 |
2015年 | 46篇 |
2014年 | 113篇 |
2013年 | 189篇 |
2012年 | 70篇 |
2011年 | 117篇 |
2010年 | 55篇 |
2009年 | 101篇 |
2008年 | 90篇 |
2007年 | 71篇 |
2006年 | 54篇 |
2005年 | 51篇 |
2004年 | 49篇 |
2003年 | 40篇 |
2002年 | 38篇 |
2001年 | 30篇 |
2000年 | 27篇 |
1999年 | 24篇 |
1998年 | 28篇 |
1997年 | 21篇 |
1996年 | 19篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1993年 | 32篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1990年 | 11篇 |
1989年 | 2篇 |
1988年 | 6篇 |
1987年 | 8篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有1584条查询结果,搜索用时 12 毫秒
111.
S. Di Maria S. Barros J. Bento P. Teles C. Figueira M. Pereira P. Vaz G. Paulo 《Radiation measurements》2011,46(10):1103-1108
The main purpose of this study was to validate and compare Mean Glandular Dose (MGD) values obtained using Monte Carlo simulations with experimental values obtained from Entrance Surface Dose (ESD) and depth dose measurements performed in a Hospital mammography unit. ESD and depth dose were measured using ThermoLuminescent Dosimeters (TLDs), and a tissue equivalent mammography phantom recommended by the American College of Radiology (ACR). Measurements and Monte Carlo simulations were also compared with the MGD calculated using the Automatic Exposure Control (AEC) system of the mammographic unit. In the simulations the Doppler energy broadening effect was also taken into account. The simulated ESD are about 5%–10% higher than the measured ESD values. The deviation between the measured and simulated MGD values in the phantom is of about 15%. The MGD evaluated using the AEC system is smaller both with respect to the Monte Carlo simulation and experimental result by a factor of about 15% and 25% respectively. Moreover the BackScatter Factor (BSF) estimated by Monte Carlo simulations was used for the MGD calculation according to the Wu’s method. Finally the inclusion of the energy broadening effect on MGD calculation produces negligible variations on the simulated results. 相似文献
112.
This paper reports on the preparation and features of a UV light dosimeter composed of nitro blue tetrazolium chloride (NBT) and polyamide woven fabric. This textile dosimeter is based on the conversion reaction of NBT into formazan, which was initially examined in aerated aqueous solutions through steady state UV irradiation. Irradiated solutions change their colour as a consequence of the formation of polydisperse NBT formazan particles. This was analysed in relation to the absorbed dose of UV light through UV–VIS spectrophotometry and dynamic laser light scattering measurements. When NBT substrate molecules are embedded in polyamide textile, UV irradiation leads to similar effects as in solution. However, the tinge intensity changes at much lower absorbed doses. The dependence of the tinge intensity on the absorbed dose was followed by measurements of the remission of light from the NBT-polyamide samples. Consequently, the calibration parameters were calculated such as the dose sensitivity, dose range, and quasi-linear dose range. An improvement of the NBT-polyamide samples by application of a colour levelling agent and improvement of their resistance to humidity is presented. Finally, the samples were used for estimation of absorbed UV energy distribution showing their capability as new dosimeters for in-plane high resolution radiation dose measurements. 相似文献
113.
Eerdunchaolu Wei Xin 《Physica B: Condensed Matter》2011,406(3):358-362
Temperature dependence of the properties of strong-coupling bipolaron in a quantum dot (QD) is studied based on the Lee-Low-Pines-Huybrechts variational method and quantum statistical theory. Results of the numerical calculation show that the vibration frequency as well as the absolute value of the induced potential and the effective potential all increase with increasing coupling strength and temperature, respectively, and they also increase with decreasing relative distance of electrons. The bipolarons are closer and more stable when the temperature is higher and coupling strength is larger. The influence of radius of QD and dielectric constant ratio on the effective potential is little. 相似文献
114.
115.
G. Cynthia Jemima SwarnavalliV. Joseph D. RoopsinghV. Kannappan 《Journal of Molecular Liquids》2011,164(3):243-249
Nanocrystallite silver and silver sol were prepared and characterized by UV-visible spectra, XRD, TEM and HRTEM. The crystallite silver is re-dispersed in two different media, namely, water and alcohol and sonicated before ultrasonic investigation. The silver sol was used as such. Three different models for the propagation of ultrasound through two phase media are compared in these three different types of nano suspensions. Effect of particle size and medium on ultrasonic velocity (U), compressibility (κ) impedance (Z) and viscous relaxation time (τ) is studied. The particle concentration range was 0.2-1 v/v. Density and viscosity of the dispersion and sol are measured at different particle volume fractions. Effective density and ultrasonic velocity are computed by Urick, Kuster and Toksöz and Urick and Ament models and compared with experimental velocities. Values of effective density obtained by using Urick and Urick and Ament equations closely agree with experimental results of density while Urick's equation prediction of velocity is in close agreement with the experimental velocities. This comes as a surprise in view of the large density difference between the medium and suspended particle and suggests the possibility of the balancing effect of the inertial and viscous forces operating in the suspension. 相似文献
116.
China Spallation Neutron Source (CSNS) is the first High Energy Intense Proton Accelerator planned to be constructed in China during the State Eleventh Five-Year Plan period, whose induced radioactivity is very important for occupational disease hazard assessment and environmental impact assessment. Adopting the FLUKA code, the authors have constructed a cylinder-tunnel geometric model and a line-source sampling physical model, deduced proper formulas to calculate air activation, and analyzed various issues with regard to the activation of different tunnel parts. The results show that the environmental impact resulting from induced activation is negligible, whereas the residual radiation in the tunnels has a great influence on maintenance personnel, so strict measures should be adopted. 相似文献
117.
莫晓虎 张建勇 张清江 Achasov Mikhail 蔡啸 傅成栋 Harris Fred 刘倩 Muchnoi Nikolay 秦庆 屈化民 王贻芳 徐金强 张天保 《中国物理 C》2011,35(7):642-655
Understanding the radiation background at the north crossing point (NCP) in the tunnel of BEPCII is crucial for the performance safety of the High Purity Germanium (HPGe) detector, and in turn of great significance for long-term stable running of the energy measurement system. Therefore, as the first step, a NaI(Tl) detector is constructed to continuously measure the radiation level of photons as background for future experiments. Furthermore, gamma and neutron dosimeters are utilized to explore the radiation distribution in the vicinity of the NCP where the HPGe detector will be located. Synthesizing all obtained information, the shielding for neutron irradiation is studied based on model-dependent theoretical analysis. 相似文献
118.
The evolution of inter-device leakage current with total ionizing dose in transistors in 180 nm generation technologies is studied with an N-type poly-gate field device (PFD) that uses the shallow trench isolation as an effective gate oxide. The overall radiation response of these structures is determined by the trapped charge in the oxide. The impacts of different bias conditions during irradiation on the inter-device leakage current are studied for the first time in this work, which demonstrates that the worst condition is the same as traditional NMOS transistors. Moreover, the two-dimensional technology computer-aided design simulation is used to understand the bias dependence. 相似文献
119.
We study higher-dimensional neutrino mass operators in a low energy theory that contains a second Higgs doublet, the two Higgs doublet model. The operators are relevant to underlying theories in which the lowest dimension-five mass operators would not be induced. We list the independent operators with dimension up to nine with the help of Young tableau. Also listed are the lowest dimension-seven operators that involve gauge bosons and violate the lepton number by two units. We briefly mention some of possible phenomenological implications. 相似文献
120.
The channel-dependent Argonne Av18 effective two-body interactions (CDEI) which are generated through the lowest order constrained variational (LOCV) calculation for asymmetric nuclear matter with the charge-dependent Av18 bare nucleon–nucleon potential are used to calculate the ground state properties of heavy closed shell nuclei such as 48Ca, 90Zr, 120Sn and 208Pb. The harmonic oscillator basis, and the local density approximation (LDA) are applied to create the relative and the center of mass dependent effective two-body potential. We get more binding with respect to the similar calculation with the Reid types potentials. It is tried to omit the LDA and perform full calculation with the Av18CDEI for light nuclei. The results indicate that the LDA works quite well. It is also shown that in case of heavy closed shell nuclei and unlike our previous report with Reid68Day interaction, the contributions of higher partial waves (J>2) are very important for the calculations with Av18 potential and we get reasonable agreement between our calculated binding energies and RMS radii, with those predicated by the others methods, and the experimental data. Finally, the various aspects of channel and density dependent two-body effective interactions are discussed. 相似文献